Industrial-based turbines are often gas-fired and are typically used at power plants to drive generators and produce electrical energy. Combustion dynamics in the combustors of such turbines are defined as pressure pulsations within the combustion system caused by feedback between non-steady heat release and combustion system characteristics. Two such characteristics are chamber acoustics and the fuel delivery system. For example, in certain combustion systems, the combustor cans are cylindrical or annular and are complex structures. It is possible to excite acoustic vibrations at one or more resonant frequencies in various elements of the combustor structure. That is, the geometry of the combustors may support multiple distinct acoustic resonances when excited. Combustion dynamics at very high levels can be very destructive and may result in the forced outage of the power plant.
The problem of combustion dynamics is known and is typically controlled to acceptable levels through a number of techniques, including geometric optimization, variation of fuel introduction location and quantity, and combustor fuel nozzle pressure ratio. For example, by changing the orientation or size of various component parts of the combustor cans or supports, the combustor system can be tuned or detuned relative to the resonant frequencies of its constituent parts. Further, by splitting the fuel delivery percentages among the various fuel valves introducing fuel into the combustor, the problem of combustion dynamics can be abated. However, these solutions require setting rigid standards for fuel gas composition and temperature.
It will be appreciated that there are a number of different types of fuel gases for the combustors of turbines, including natural gas, LPG's such as propane and butane, refinery gases and coal-derived gases. The energy content of each of these fuels varies with its source and, of course, there are variations in energy content among the various types of fuels. The temperature of the fuel gas supplied to the combustor can also be quite different from system to system. For example, many power plants generating electricity from the output of gas turbines provide a fuel gas heater to provide a constant fuel gas temperature to the combustor. Other sites may have a number of boost compressors to elevate the temperature. Thus, different sites provide fuel gas at different temperatures and pressure. Furthermore, several sites source fuel gas from several different vendors which implies that both the temperature and composition of the fuel gas can vary.
The standards for setting fuel gas composition and temperature are defined by a parameter called the Wobbe Index. The Wobbe Index allows comparison of the volumetric energy content of different fuel gases at different temperatures. Since the gas turbine reacts only to energy released in the combustors and the fuel flow control process is actually a volumetric flow control process, fuels of different composition with relatively close Wobbe Indices can generally be provided in the same fuel control system. The Wobbe Index is defined most generally as the relative fuel heating value divided by the relative density. More particularly, the Wobbe Index is: ##EQU1## where: WI=Wobbe Index; LHV=Lower heating value (BTU/scf);
T.sub.g =Absolute temperature; PA0 SG=Specific gravity relative to air at STP (Standard Temperature and Pressure).
Allowable variations in Wobbe number are often specified as less than .+-.5%. However, variations in Wobbe Index from the specified value can lead to unacceptable levels of combustion dynamics. That is, it has been determined that combustion dynamics are a function of the Wobbe Index. Consequently, operation at high levels of variations in the Wobbe Index from a specified value can result in hardware distress, reduced component life of the combustion system and a potential for power generation outage.